The present invention relates to a liquid crystal display apparatus, wherein an electric field is supplied in a direction parallel to the surface of the substrate, and more particularly to an active matrix type liquid crystal display apparatus which provides a wide viewing angle compatible with a high image quality.
In a conventional liquid crystal display apparatus, an electrode for driving a liquid crystal is formed on the surface of each of two substrates, respectively, so that the electrodes are facing each other across the liquid crystal. The above conventional liquid crystal display apparatus uses a method which is represented by a twisted nematic display mode, wherein the liquid crystal is driven by supplying an electric field in a vertical direction with respect to the two substrates. In such case, transparent electrodes, such as ITO (Indium Tin Oxide), are used.
On the other hand, another mode of operation, wherein the liquid crystal is driven by supplying an electric field in a direction approximately parallel to the substrate using comb shaped electrodes provided on one substrate, is disclosed in JP-B-63-21907 (1988), and U.S. Pat. No. 4,345,249. In this case, the electrodes need not necessarily be transparent, and opaque metallic electrodes having a high electric conductivity can be used.
However, in the display mode wherein the electric field is supplied to the liquid crystal in a direction approximately parallel to the substrate using active elements (hereinafter called an in-plane switching mode), any method to supply an adequate electric field to the liquid crystal without causing interference between the electric fields supplied in a direction parallel to the substrate in a condition where a large amount of wiring exists, and any means to concurrently improve the image quality, have not been disclosed entirely.
Generally speaking, in the in-plane switching mode, the electric fields interfere with each other, because a large amount of wiring is formed on only one of the substrates through which electric signals are transmitted. Accordingly, the electric field supplied to the liquid crystal is influenced by unnecessary electric fields, and so an adequate electric field can not be supplied to the liquid crystal.
Furthermore, an unnecessary electric capacitance is formed between the electrodes, which sometimes causes the voltage supplied to the liquid crystal to fluctuate. The above described phenomena will cause deterioration of the image quality of the liquid crystal display apparatus. Especially, an electric field generated by an image signal electrode for transmitting signals to respective pixels having an active element, such as a TFT and the like, influences the electric field between a pixel electrode for operating the liquid crystal and the common electrode.
The potential of the image signal electrode varies always during the frame period in the course of transmitting signals. It has been known that, if the pixel electrode (its potential is in a floating condition when the active element is in an off condition) is located close to the image signal electrode, a nonuniformity appearing like shadow stripes referred to as a smear, similar to a cross talking phenomenon, is generated in parallel with the image signal electrode depending on the varying potential of the image signal electrode. In order to suppress this phenomenon, a technique to arrange the common electrode, which is always supplied with a potential from an external source, as the closest electrode to the image signal electrode has been developed by the present inventor (U.S. patent application Ser. No. 08/374,531).
However, in accordance with the above technique, the shielding effect for the electric field is not necessarily sufficient, and the problem caused by generation of the smear phenomenon still exists. Although the smear phenomenon could be suppressed by increasing the shielding effect with broadening of the width of the common electrode, the broadening of the width of the common electrode causes another problem in which the aperture ratio of the liquid crystal display apparatus is decreased.
One of the objects of the present invention is to solve the above problems, and to provide an active matrix type liquid crystal display apparatus of the in-plane switching mode type, which is capable of providing a wide viewing angle and a high image quality without generating the smear phenomenon.
The gist of the present invention to achieve the above object is as follows.
An active matrix type liquid crystal display apparatus, comprises a plurality of electrodes, which are formed on a substrate so that an electric field in parallel to the substrate can be supplied to a liquid crystal layer, and a polarizer is provided, which changes its optical characteristics based on an alignment condition of the liquid crystal layer, wherein a shielding layer formed on the substrate in parallel to the image signal electrodes has a specific resistivity of less than 108 xcexa9.cm.
The shielding layer formed on the substrate in parallel to the image signal electrodes is desirably further coated with an insulator of at least 108 xcexa9.cm.
The specific resistivity of the shielding layer formed in parallel to the scanning electrode is also desirably at least 108 xcexa9.cm.
A counterpart to an image signal electrode of the shielding layer formed in parallel to the image signal electrodes is preferably an electric conductor, and the potential of the shielding layer formed in parallel to the image signal electrodes is preferably set at the same level as the potential of the common electrode.
This means that a member for absorbing an electric field is used as the shielding layer concurrently, so that the electric field generated from the image signal electrode is shielded so as to exert no influence on the electric field supplied to the liquid crystal layer. Further, the member for absorbing the electric field may be arranged at only a counterpart to the image signal electrode, and a high shielding effect is generated by arranging the shielding layer made of a high insulating material, such as a metal, so as to cover the member.
The member for absorbing the electric field can be made of an electric conductor, such as a metal. In this case, the shielding effect for the electric field can be increased by arranging the member for absorbing the electric field at only the counterpart to the image signal electrode, providing the member so as to have the same potential as the potential of the common electrodes, and arranging the shielding layer having a high specific resistivity so that it covers the member.
As the insulating and shielding layer, an organic polymer material mixed with conductive particles, such as metallic particles and/or carbon particles, is used, and its specific conductivity can be controlled by adjusting the mixing amount of the conductive particles.
A theory of operation of the in-plane switching mode will be explained hereinafter.
FIG. 1 indicates a schematic structure of an active matrix type liquid crystal display apparatus of the in-plane switching mode type, relating to the present invention. A feature is that the composition of the black matrix 22a, which is parallel to the image signal electrodes, differs from the composition of the black matrix 22b which is parallel to the scanning electrodes.
FIGS. 2(a) and 2(b) are schematic vertical cross sections indicating the operation of the liquid crystal in the liquid crystal panel of the present invention, and FIGS. 2(c) and 2(d) are their schematic elevations. A vertical cross section of the cell when no voltage is supplied is indicated in FIG. 2(a), and its elevation is indicated in FIG. 2(c). Active elements are omitted in these figures. In accordance with the present invention, plural pixels are formed by forming plural stripe shaped electrodes, but only a part of the pixels are shown in FIGS. 2(a) and 2(b).
FIG. 3 indicates a relationship between an angle xcfx86p formed by the polarized transmission axis 11 of the polarizer 8 to the direction of the electric field 9, and an angle xcfx86LP formed by the longitudinal axis (an optical axis) of the liquid crystal molecule (the rubbing direction 10) at the vicinity of the substrate boundary to the direction of the electric field 9. The polarizer and the substrate form a pair at the upper region and lower region, respectively. Therefore, if necessary, the symbols xcfx86P1, xcfx86P2, xcfx86LC1, xcfx86LC2 are used.
As shown in FIG. 2(a), stripe shaped electrodes 1, 3, 4 are formed inside of a pair of transparent substrates 7, 7xe2x80x2, whereon alignment layers 5, 5xe2x80x2, are formed, and a liquid crystal layer is interposed between them.
A rod shaped liquid crystal molecule 6 is aligned so as to form an angle to the longitudinal direction of the stripe shaped electrodes 1, 4, that is, 45 degrees  less than |xcfx86LC|xe2x89xa690 degrees, when no voltage is supplied. In the following explanation, a case when the alignment directions 10 of the liquid crystal molecules 6 at the upper boundary and the lower boundary are in parallel, that is when xcfx86LC1=xcfx86LC2, is taken as an example. The dielectric anisotropy of the liquid crystal composition is assumed to be positive, but, even if it is negative, no problem results.
By supplying an electric field 9, the liquid crystal molecules change their alignment direction along the direction of the electric field, as shown in FIGS. 2(b) and 2(d). Therefore, the optical transmissivity can be altered by supplying an electric field while arranging the polarizers 8, 8xe2x80x2 at a designated angle (direction of the polarized transmission axis 11).
When the dielectric anisotropy of the liquid crystal composition is negative, the initial alignment direction is oriented to an angle |xcfx86LC|, which is perpendicular to the longitudinal direction of the stripe shaped electrode, (that is, 0 degree  less than |xcfx86LC|xe2x89xa645 degrees).
A means for shielding unnecessary electric fields relating to the present invention will be explained hereinafter.
Smear, a cross talk phenomenon, can be decreased by restricting the specific resistivity of the black matrix 22a, which is in parallel to the image signal electrodes, to less than 108 xcexa9.cm.
In accordance with the in-plane switching mode, the electrodes are formed fundamentally only on the substrate whereon the active elements are mounted. And, because the electric field is supplied in a direction parallel to the substrate, any conductor on the opposing substrate becomes a disturbance to the electric field.
Therefore, when a black matrix is provided on the opposing substrate, an insulator, not a metal, which does not influence the supplied electric field, has been used as the material for the black matrix.
The inventor found that the shielding effect, which ensured that the unnecessary electric field between the image signal electrodes 3 and the pixel electrodes, and between the pixel electrodes and the common electrodes 1, would not influence the electric field parallel to the substrate between the pixel electrodes and the common electrodes 1, could be obtained by restricting the specific resistivity of the black matrix formed in parallel to the image signal electrode 3 to less than 108 xcexa9.cm. The above finding is based on a relationship that an insulator having a specific resistivity less than 108 xcexa9.cm absorbs an electric field even though it is an insulator.
The relationship is shown in FIG. 4(a). The relationship indicates a driving voltage at a spot (b), where the influence of the black matrix could be expected, when the black matrix is formed on the substrate (II) counter to the substrate (I) having comb-shaped electrodes thereon, as shown in FIG. 4(b).
As a result, it was revealed that the driving voltage increased in a range wherein the specific resistivity of the black matrix material was less than 108 xcexa9.cm. This means that an insulator having a specific resistivity less than 108 xcexa9.cm could absorb an electric field. Accordingly, a shielding effect with respect to the electric field, which is generated by the image signal electrodes and is unnecessary for driving the liquid crystal layer, can be obtained by using an insulator having a specific resistivity less than 108 xcexa9.cm as the material for the black matrix. And, an improvement in preventing the generation of cross talk, appearing as a smear generated in parallel to the image signal electrodes, can be achieved.
However, there are several practical methods-in setting the specific resistivity to less than 108 xcexa9.cm for the black matrix formed counter to the image signal electrodes in order to shield unnecessary electric fields of the image signal for preventing generation of cross talk One of the methods is that the entire black matrix formed counter to the image signal electrodes is fabricated with a material having a specific resistivity which is less than 108 xcexa9.cm. However, in this case, the possibility exists that a displacement in alignment of the upper and lower substrates may occur, causing the black matrix to intrude into a liquid crystal driving region. In such a case, a problem may occur in that an adequate electric field can not be supplied to the liquid crystal layer. The problem relating to the alignment can be solved by making only a part of the black matrix formed counter to the image signal electrodes, especially only the part aligned with the image signal electrodes, have a specific resistivity less than 108 xcexa9.cm, and providing the rest of the part with material having a specific resistivity at least 108 xcexa9.cm. The above double structure can be replaced with a structure wherein the entire structure made of a material having a specific resistivity less than 108 xcexa9.cm is covered with a material having a specific resistivity of at least 108 xcexa9.cm, as shown in FIG. 7. Further, a structure wherein the material having a specific resistivity less than 108 xcexa9.cm may be interposed between the material having a specific resistivity of at least 108 xcexa9.cm, as shown in FIG. 8. Furthermore, a structure wherein a concave shaped material having a specific resistivity less than 108 xcexa9.cm may be fit in a convex shaped material having a specific resistivity at least 108 xcexa9.cm.
The shielding effect with respect to the electric field of the black matrix formed in parallel to the image signal electrodes can be increased further by using an electrically conductive material as the material having a specific resistivity less than 108 xcexa9.cm.
In the above case, the absorbing effect with respect to the electric field can be more properly stabilized when the conductor has a constant potential, than in a case when the conductor has a drifting potential. Accordingly, the shielding effect with respect to the electric field can be increased further by setting the potential of the conductor always at the same level as the potential of the common electrode 1.
The specific resistivity of the above materials, wherein metallic particles or carbon are dispersed in an organic high polymer material, can be adjusted by controlling the amount of metallic particles or carbon in the material.